This invention relates to a method and apparatus for the edge detection of transparent films, specifically, for detection by means of polarized light of position and shape of the edge of transparent films with the property of rotatory polarization.
The edge position of web like opaque films has been detected with photoelectric sensors in response to the interception by the web of irradiated rays on the area of the edge. For transparent webs the pneumatic devices have been employed, as described in the references (see, for example, "Functions of present-day sensors and their optimal exploitation," Gijutsu-Hyoron-sha, 1981, pp. 65-67). FIGS. 4A and 4B illustrate a mechanism in the devices. In FIG. 4A the device controls the movement of web edge at a desired position in winding or unwinding a web. The edge of web is placed inside of the sensing nozzle of U-shape, and the low-pressure air (200.about.400 mm H.sub.2 O) is injected to the lower side of the web, which interrupts the air flow to change the recovery pressure to the sensing nozzle. The extent of interruption is dependent on the edge position of web so that the relationship of the displacement of the edge with the recovery pressure is as shown in FIG. 4B. The change in recovery pressure is transformed through the diaphragm into the displacement of the spool, which controls the hydraulic cylinder.
The optical methods like photoelectric sensors cannot be applied to detection of the edge of transparent films. The pneumatic methods, however, are less satisfactory than the optical methods with respect to accuracy and response in detection. The pneumatic devices also require clean air as well as ever clean environment to keep the ambient dust from suction.